CRUSTAL CONTAMINATION IN THE TERMINAL STAGES OF THE KEWEENAWAN LARGE IGNEOUS PROVINCE: GEOCHEMICAL EVIDENCE FROM THE PORCUPINE MOUNTAINS

Large igneous provinces and associated flood basalts are thought to be the surface expressions of upwelling mantle plumes, but persistent questions remain about how these magmatic provinces behave in their final phases. The 1.1 Ga Keweenawan large igneous province, which erupted into the contemporaneously developing North American Mid-Continent Rift, preserves the most complete record of plume-related volcanism from the Proterozoic. The Lake Shore Traps (1085 Ma), located in the Porcupine Mountains of Michigan, represent some of the youngest mafic material erupted in the Keweenawan province. The Lake Shore Traps are separated from the main-stage Portage Lake Volcanics (1096-1093 Ma) by a layer of conglomerate, indicating a distinct final pulse of renewed magmatism after an approximately 8 million-year hiatus. Thus, the geochemistry of these rocks can aid in our understanding of the waning phases of large igneous provinces. We report that the Lake Shore Traps exhibit among the most negative ε_Nd and ε_Hf values (n = 3; ε_Nd -9.4 to -5.9; ε_Hf -15.7 to -14.7) observed in the Keweenawan large igneous province. These isotopic signatures are consistent with the incorporation of significant volumes of crustal material into Lake Shore Traps magmas. These signatures contrast with those of the Portage Lake Volcanics, which have ε_Nd values near zero that represent the Keweenawan plume. Trace element spider diagrams of the Lake Shore Traps and Portage Lake Volcanics exhibit patterns that are largely parallel but with the Lake Shore Traps elevated to more enriched values. The slopes of the rare earth element patterns are elevated for the Lake Shore Traps ((Ce/Yb)_N ~5.2) compared to the Portage Lake ((Ce/Yb)_N ~3.1). We interpret the trace element data to indicate the Lake Shore Traps magmas had the same source as the Portage Lake, but at a reduced degree of partial melting and with an increased residence time in the crust. This difference suggests a decreased magmatic flux during renewed magmatism. Taken together, the isotopic and trace element characteristics of the Lake Shore Traps suggest waning thermal and magmatic input from the Keweenawan plume during this final pulse of magmatism.